1. Field of the Invention
The present invention relates to spread spectrum communication systems using PN coding techniques and, more particularly, to a method and system for generating and detecting variable sequence PN codes.
2. Description of Related Art
Spread spectrum (SS) systems, which may be CDMA systems, are well known in the art. SS systems can employ a transmission technique in which a pseudo-noise (PN) PN-code sequence is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver, the signal is de-spread or acquired using a synchronized replica of the PN-code sequence.
In general, there are two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the carrier frequency and Rc represents the PN-code chip rate, which in turn may be an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver sharing a common PN code sequence or having its own designated PN-code. Although each receiver receives the entire frequency bandwidth, only the signal with the receiver's matching PN-code sequence will appear intelligible; the rest appears as noise that is easily filtered.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time, a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2n frequency positions. The receiver follows the same frequency hop pattern.
Most often the PN-code sequences are very long codes, which deny an intercepting receiver the ability to determine the sequence and exploit it.
Systems employing a hailing signal burst often call for a receiver to acquire and receive data within a short time frame, e.g., two milliseconds. To detect a preamble quickly upon arrival at a receiver, a large parallel correlator may be used to search for the full PN sequence. To satisfy low latency requirements on the hailing channel, using the same PN sequence at the beginning of each hail makes detection and acquisition of the hail easier, since the parallel correlator can always be searching for a fixed sequence. However, a short, repeated PN code is vulnerable to an intercept receiver.
Other options for PN-code sequences include free running PN codes in which the PN code continually runs (i.e., a long PN code) whether a burst hail is present or not. Searching for a free running PN-code is difficult because the range or uncertainty of communication range uncertainty increases the search window thus increasing the noise level in the detection receiver. Furthermore, free running codes require a prohibitive increase in hardware necessary for searching each possible phase of the preamble sequence.
It is therefore an object of the present invention to provide a method and system for generating PN-code sequence changes periodically at a rate fast enough to avoid exploitation by an interceptor of a burst hailing channel, while maintaining a suitable correlation search window.
It is a further object of the present invention to provide a method and system for generating PN-code sequence changes at a slower rate than the chipping rate.
It is still a further object of the present invention to provide a method and system that allows for fast acquisition time of a burst signal without a prohibitive growth in hardware.